Well, this is where your patience really does become your virtue!
Mount, remount. Mount, remount. When he moves, correct him and try again. Again and again. Untill he stands still. Then let him have a break.
As frustrating as it is, he will eventually clue in that the sooner he stands still, the sooner he can you stop coming up and off him.
I was training this paint gelding, Mickee, who used to bolt everytime I got on and dismounted (for awhile I literally had to bail off while he loped away, which was interesting). Seems like a bad thing to do, but I fought with him constantly. So mounting up I let him run. When he didn't want to run anymore I asked him to lope a few more circles, asked him to whoa. He was tired enough to only dodged away.
Eventually, about a week later, he figured out that running wasn't such a good idea after all, and he never once moved again while mounting or dismounting. He figured out, on his own without a fight, that his idea sucked and mine worked.
I hate it when I get on a horse and they move without me asking. The horse I'm training right now is like that. Stop her, back her to where she was, say whoa. Correct her, whoa. Correct her, whoa.
She drives me insane but we are getting there, slowly but surely.
Good luck with your boy! :)